1. Field of the Invention
Packagings for X-ray film protect the photographic material in particular from light ingress, moisture and mechanical damage, as well as--in the case of dental X-ray films--contact with the saliva of the patient.
A packaging material for X-ray films must meet these functional requirements while being as economical as possible to process into the packaging units. These are generally constructed as envelopes or bags made by joining film strips and bonding them at the edges by welding, glueing or sealing. They enclose the material, the total film package comprising the actual film and any further inserts, such as for example cardboard or lead foil inserts.
2. Description of the Prior Art
GB-PS 1 098 950 relates to the construction of such a bag, and in particular names as a packaging material soft PVC film which as a result of appropriate selection of thickness and coloration offers the necessary light-shielding, and as a result of its being able to be thermally-welded also offers a simple way of bonding the components of the bag.
In addition to polyethylene (PE), GB-PS 020 183 also names in particular PVC as a suitable packaging material for X-ray films which are, however, in this case particularly utilised for the non-destructive testing of pipe welds.
Soft PVC films for the manufacture of dental X-ray film packages are widely distributed commercially and as a rule possess two layers of the same thickness, one of which is white and the other of which is black or coloured. The white layer forms the exterior of the pack and predominantly signals that the product belongs to the "white", or medical, sector. As a subordinate function it contributes to the light-shielding properties of the film. This function is fulfilled in the main by the black or coloured layer.
Since these PVC films modified for softness exhibit comparatively low mechanical strength, they are used in relatively great thicknesses of 170 to 200 .mu.m. Only in this way is it possible to protect the film material adequately from mechanical damage from external sources. This great thickness is furthermore necessary in order to protect the film itself from being damaged by the sharp-edged photographic film material or other package components which are frequently used such as, for example, lead foil and cardboard inserts.
EP-PS 0 316 520 relates to a dental X-ray film package and explains that insertion of the package into the oral cavity of the patient is often felt to be unpleasant, if the X-ray film package is too rigid. Hard edges of the package add to the discomfort.
EP-PS 0 316 520 therefore recommends a soft or flexible material, such as for example polystyrene (PS), for manufacturing of the dental X-ray film package therein described, and in particular draws attention to the fact that selection of a packaging film which is markedly softer or more flexible than the contents of the package, that is to say photographic film material and inserts, can appreciably reduce the patient's discomfort.
For this reason, therefore, a packaging film should be sought which is of a material of great softness. The material should simultaneously possess a high degree of strength, including resistance to puncture, so that the film has a high degree of flexibility (may be as thin as possible).
EP-A 0 363 092 relates to a dental X-ray film package the structure of which minimises the risk of contact between the saliva of the patient and the photographic film material, and furthermore to a method which reduces the probability of transmission of pathogenic bacteria contained in the saliva between the patient and the operator taking the X-rays.
The first step in this method comprises sealing the photographic film material in a bag of a material which is transparent to X-rays but impermeable to saliva and microorganisms. These are further important properties of a film for packaging X-ray films.
EP-A 0 363 092 relates furthermore to the construction of an X-ray film package having the aforementioned purpose, wherein the package itself is of a thermoplastic, thermally-weldable material.
Packages of polyethylene (PE), polypropylene (PP), polycarbonate (PC), cellulose propionate and cellulose acetate are explicitly protected.
Films out of these plastics admittedly meet the requirements of impermeability to saliva and microorganisms, but they are already harder or more rigid than conventional commercial soft PVC films, and thus they do not achieve the required user-friendliness for the patient. Some, furthermore, exhibit deficiencies as regards resistance to abrasion and mechanical damage.
In order to increase the strength of the packaging material and increase its resistance to puncture, JP-PS 199 393, JP-PS 098 805, JP-PS 267 775 and EP-A 0 276 018 propose multilayered film structures.
Thus, for example, JP-PS 199 393 relates to a laminate having at least four layers comprising a porous layer, for example a mesh out of a thermoplastic polymer, a flexible layer, a layer of foamed plastics and a thermoplastic sealing layer.
In addition to the relatively high cost of production, a deficit of this structure lies in its poor material recyclability. For as a result of efforts to return used packaging materials and put them to new uses, the mechanical recyclability of a material is a very important additional requirement alongside its technological properties.
JP-PS 098 805 relates to a packaging material for films the manufacture of which involves bonding a flexible thermoplastic layer to both sides of a mesh of a thermoplastic. The raw materials named for the mesh are HDPE, LLDPE or PET, and PE, PP or PA for the flexible layers. Besides the fact that this structure is relatively rigid, it is only when, exceptionally, both the mesh and the flexible layers utilised are of PE that it can be reused without problems as material of the same kind.
JP-PS 276 775 describes the construction of a package for photosensitive photographic material and the packaging material itself. It comprises a multilayered co-extruded film of which one outer layer consists of an ethylene copolymer and the other of a polyolefin. As polyolefins PE, PE/PP copolymers, PE/PB copolymers, PP/PB copolymers, PB, PS, PMMA or PP may be used. Under some circumstances a central layer may also be co-extruded between the two covering layers.
EP-A 0 276 018 also relates to a multilayered packaging material for photosensitive materials, wherein once again one of the layers comprises an ethylene copolymer. The second layer is composed of a thermoplastic, has a modulus of elasticity greater than 40 kg/mm.sup.2 and is less hygroscopic than the ethylene copolymer layer. A flexible layer, which may comprise a plastic, including a metallised plastic, paper, metal foil or for example also a non-woven fabric, is laminated on to the second layer.
The latter two patents therefore describe packaging materials which, because different materials like plastics, paper and metal and composite materials thereof are utilised , do not meet the requirements of mechanical recycling. As a rule they also fail to meet the requirements for softness and flexibility.
A further critical disadvantage of such packaging structures is that ethylene copolymers which are modified with vinyl acetate, acrylates, such as for example ethyl acrylate or methyl acrylate or acrylic acid, are not neutral in odour. The typical, acidic odour of such plastics is felt by the majority of people to be unpleasant.
A two-layer coextruded film of LDPE and HDPE, as described in JP-PS 229 500 as a packaging material for photosensitive material, is well suited to material recycling but is inadequate as regards softness and flexibility.
The packaging film for photographic films in accordance with JP-PS 021 614, a biaxially orientated PP film with PVDC coating, also exhibits a high degree of rigidity.
DE-PS 632 041 and GB-PS 056 848 in each case relate to a packaging for a stack of film material, which facilitates the insertion of the stack into a supply cassette which is part of the X-ray apparatus. For this application the softness and flexibility of the packaging film are of less importance. The three-layer packaging material structures described therein--PE sealing layer/opaque core layer/high-strength layer, preferably polyester or paper/aluminium foil/PE--are accordingly also of a rigidity which is too great for a dental X-ray film package.
The object of the present invention consequently lay in the provision of a packaging film of the greatest possible softness and flexibility for photosensitive material, in particular dental X-ray films which, however, simultaneously excelled in good mechanical strength, low abrasion and good resistance to mechanical damage. The film should additionally exhibit light-shielding properties, but be transparent to X-rays, impermeable to saliva and microorganisms and neutral in odour and taste. Further requirements of the packaging material were economical processability when forming the packaging units on packaging machinery and its ability to be welded under the influence of pressure and heat or of a high frequence field. The packaging material should exhibit improved mechanical properties to enable a reduction in the required film thickness, and hence also a reduction in the proportion of the product comprising packaging, and should be recovered by mechanical recycling after use.
Surprisingly, it was possible to achieve this object by manufacturing a multilayered film based on thermoplastic polyurethane (TPU).